A bit late for me to come into this discussion, but I always thought that the alien that he sees in the field, and the one they chase around the house, is just a scout, so would not attack. They check out the area and report back on things like numbers, resistance, layout, possible attack points or defensive positions, that kind of thing. They don't attack as they need to get their butts back to base so they can pass on that information.

And yeah, with the gas question I always thought it was poison and that his asthma saved his life as he did not manage to breath any of it in (have a listen to the dialog of Graham when they are outside hoping he will wake up).

As Officer Paski walks back to her cruiser, her ballpoint pen shifts from being clipped on the outside of her shirt pocket to being fully inside.

The basement door is green in every shot except the very last one when Merrill is opening the door to go up and check for the "all clear" - it is a close-up of the handle and the door is painted brown.

Merrill disappears from the end of the table when Morgan is talking to the sheriff.

When Graham comes to the children after Morgan had to kill their dog, Bo is sitting with her knees up to her face. In this shot her shoes are perfectly tied. Once it changes to Graham and Morgan looking down at the dead dog and Morgan leaves, Graham goes over to Bo. Her right shoe lace is untied.

When Graham looks out the window at the large flock of birds, the corn stalks are brown. This is in contrast to all of the other shots, were the corn is green.

Audio/visual unsynchronized:

Graham asks Bo to turn up the volume on the TV, she reaches forward and the volume increases before she makes contact with the button. Also, she then holds her finger on the button but the volume does not continue to increase, instead, it remains constant.

Crew or equipment visible:

Crew members, one possibly holding a boom mike, are reflected in the lights of Caroline's police car when she is talking to Graham after they have seen the crop circles on TV for the first time.

When Graham is at Reddy's house, Reddy speeds away in his truck the camera is reflected in the window.

As Graham takes Bo inside, after Houdini has died, they meet Merrill on the way into the house. As Bo mutters "Houdini's sick", the shadow of a boom mike can be seen traveling across Merrill's shirt and face.

Revealing mistakes:

The family television has no problems picking up a clear TV signal, even though the antenna cable isn't plugged in.

Factual errors:

The man on the Army poster in the recruiting office is wearing a US Marine Corps uniform.

The aliens come out of the corn fields at night. Their severely painful reaction to water would prevent this as corn leaves collect a large amount of dew. Walking through a corn field late at night could leave you pretty soaked.

That's a good point, but still, when you make something you want it to be the best possible.

And maybe that's why some of the mistakes are there... right? Maybe Night decided to use the take with the best performance from Officer Paski (Cherry Jones) even though her pen shifts from being clipped on the outside of her shirt pocket to being fully inside.

There is that small continuity mistake, but the trade of is a superior and more believable performance... (hypothetically in this case... probably )